i40e/i40evf: Fix compile issue related to const string

i40e/i40evf: Fix compile issue related to const string

Orabug: 22342532

Add const to functions that return strings that aren't going to be
modified. This addresses some reported compile complaints.

Change-ID: Ic56b1e814ab4d23a50480e7fdec652445f776ee8
Signed-off-by: Jingjing Wu <jingjing.wu@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
(cherry picked from commit 4e68adfeb91d684e5c7a0c6305af872120c71f23)
Signed-off-by: Brian Maly <brian.maly@oracle.com>